WASHINGTON – Efforts to develop the world's first vaccine to prevent cervical cancer took a key step forward yesterday with test results suggesting that it can provide long-lasting protection.

Four years after getting the vaccine, 94 percent of women were protected from infection with the virus that causes most cervical cancers and none had developed worrisome precancerous conditions, a study showed.

"We're thrilled about these results. The immune responses seem to be really long-lasting," said Dr. Eliav Barr, who leads development of the vaccine for Merck & Co.

Merck plans to seek Food and Drug Administration approval next year for an expanded version of the vaccine that also could be used to prevent genital warts in women and men.

The new study was funded by Merck and led by University of Washington researchers who presented results yesterday at a meeting of the American Society for Microbiology.

"They showed clear effectiveness," said Dr. Scott Hammer, a Columbia University infectious disease expert who reviewed the work but has no ties to Merck or the study. "This is a very important issue for women's health around the world."

If the vaccine makes it to market it would be the second developed to prevent cancer. The hepatitis B vaccine has dramatically reduced the number of infections that progress to liver cancer.

Cervical cancer strikes nearly a half-million women worldwide each year and kills about half. In the United States, about 15,000 women get it and about 5,000 die.

The vaccine works against viruses that belong to a group called human papillomaviruses, or HPV. Nearly every case of cervical cancer is caused by HPV. The viruses are sexually transmitted, extremely common and almost impossible to avoid.

More than 30 types of HPV infect the human genital area. Only some types cause cancer; others cause genital warts. A type known as HPV-16 causes 50 percent of cervical cancers, and HPV-18 causes 20 percent. Other types cause the rest.

A previous study showed that HPV-16 infections were completely prevented in 768 women who had received the Merck vaccine 18 months earlier. None developed precancerous conditions.

The new study followed 755 of these women for four years after vaccination. HPV-16 infections had taken hold in seven; none developed precancers. In a comparison group of 750 women who received dummy shots, infections took hold in 111 and precancers formed in 12.

Though protection had waned for a small number of women in the study, the vaccine's effectiveness still was very high, said Dr. Douglas Lowy, a National Cancer Institute scientist who invented the vaccine. The government gave rights to develop it to Merck and GlaxoSmithKline, and Merck's work is a little further along, Lowy said.

"Revaccination might be advisable at some point," but it will take more study to know whether that would improve effectiveness, he said.

Women in the study were ages 16 to 23 when they received the vaccine, given in three doses over six months.

"Most people think it would be recommended for young adolescents," Lowy said. "The idea would be you would immunize people relatively soon before they become sexually active."

Merck is in the final stages of testing an expanded vaccine that contains HPV-16 and HPV-18, as well as strains that cause genital warts in men and women, and penile and anal cancers in men.

About 25,000 women and men have been enrolled in that study in 34 countries, and results are expected next year, Barr said.

Giving the vaccine to men not only would prevent disease in them but also would prevent infections in their partners, said Steven Projan, a drug development expert with Wyeth who helped review research for the microbiology meeting. The vaccine also might prevent women already infected with HPV from developing cancer, Projan said.